The disclosure relates in particular to an angle superposition device with servo drive for a steering system for non-trackbound motor vehicles which superposes the rotational speeds of the servo drive and the control actions input by the driver at the steering wheel and transmits and converts these into the steering movement of the wheels. The expressions “rotational angle superposition device”, “steering angle superposition device” or “rotational speed superposition device” are used synonymously for the expression “angle superposition device”.
A range of such systems are known in the prior art, wherein the rotational speed superposition gearing of the angle superposition device is realized by way of a planetary gear set, or else additionally has a worm gear mechanism.
DE19823721A1 proposes an angle superposition device of said type. In this case, by way of a steering wheel, a housing is driven in which toothings of an internal gear of two planetary gear sets are situated. An electric motor arranged in the housing drives the sun gear of the first planetary gear set. The planet carrier of the first planetary gear set drives the sun gear of the second planetary gear set. The planets of the second planetary gear set are supported on the internal gear of the housing, and the planet carrier is connected to the output shaft. In the embodiment shown, the drive of the sun gear of the first planetary gear set is realized directly by way of the rotor of the electric motor. Through corresponding control of the electric motor, the desired rotational speed superpositions can be realized. Said solution which is presented in the prior art however has some significant disadvantages. Such gearings are composed of a multiplicity of components. Two internal gear rings or planetary gear sets are required. The production of the individual parts is highly complex and expensive. The multiplicity of points of meshing engagement leads to very high accuracy requirements in order to avoid play. Furthermore, the entire device, including motor housing, must be jointly rotated by the driver during steering actions. A further disadvantage furthermore consists in the cumbersome coupling-in of the electrical energy for the electric motor.
In further prior art, DE19852447A1, a solution for rotational speed superposition is proposed in which an electric motor is coupled by way of a worm drive to the superposition gearing, which is in the form of a planetary gear set. In this case, the gearing unit is arranged fixedly with respect to a body. However, this solution is also associated with a series of disadvantages. The coupling-in by way of a worm gearing leads to very low levels of efficiency of the rotational speed transmission. Furthermore, the arrangement requires a considerable amount of installation space, with little flexibility owing to the geometrically defined positions of the components with respect to one another. In this case, too, the production of the individual parts is complex and expensive and the demands with regard to production tolerances is correspondingly high.